1. Field of the Invention
The present invention relates to an acousto-optic modulator and a manufacturing method thereof, and more particularly, to an acousto-optic modulator comprised of an acousto-optic device coated with an anti-reflection layer and a manufacturing method thereof.
2. Description of the Related Art
The acousto-optic systems currently in use are comprised of an optical source, an acousto-optic modulator (AOM), and an optic system. Here, the optical source generates a second harmonic frequency being a continuous wave in the bluish-green area. The AOM controls the second harmonic frequency by diffraction by applying the acousto-optic effect. The optical system is arranged between the optical source and the AOM and controls laser.
A second harmonic oscillator, which is pumped by a laser diode emitting a bluish-green light, is an optical source which is very useful for high-density photomagnetic recording. A bluish-green continuous wave laser must be modulated in accordance with an input signal to be used in an information recording/reproducing apparatus used in an audio/video system. The AOM is used for such modulation. The AOM is an optical device using the acousto-optic effect, is a high-speed optical switch, and is being widely used as an optical output modulation component.
FIG. 1 is a perspective view of an exemplary AOM structure.
Referring to FIG. 1, in the AOM, a transducer 12 for generating an acoustic elastic wave by electrodes 14A and 14B is installed at one side of an ultrasonic medium 10. Also, an acoustic elastic wave absorbing element (not shown) for preventing the influence of reflection of an ultrasonic wave is provided on the opposite side of the ultrasonic medium 10. The ultrasonic medium 10 is made of a material selected from the group consisting of fused quartz, PbMoO.sub.4, TeO.sub.2, Te glass, and Schwer-Flint glass (SF.sub.4). Among them, PbMoO.sub.4 or TeO.sub.2 crystal material is used for relatively high frequencies, and glass materials are used mainly for low frequencies since it is cheap but has a large propagation loss at a high frequency. Light Incident/emitting surfaces 10A and 10B through which laser light passes are optically polished. On account of a great refractive index of the ultrasonic medium 10, there is a large reflection loss when laser light enters or is emitted.
For example, when TeO.sub.2 single crystal is used as a constituent material of the ultrasonic medium, light absorption into the TeO.sub.2 single crystal medium and light reflection on the surface thereof cause a reduction of about 25% or more in transmission of laser light when an anti-reflection layer is not coated on the light incident/emitting surfaces. A high-density optical information processor such as a digital video disk recorder (DVDR) requires use of a high output laser. Therefore, when the TeO.sub.2 single crystal is used as a constituent material of the ultrasonic medium, the anti-reflection layer should be coated to increase the efficiency of the output power of a laser light.
In the prior art, an Al.sub.2 O.sub.3 layer has been used to form the anti-reflection layer to be coated on the light incident/emitting surfaces of the ultrasonic medium composed of TeO.sub.2 single crystal. The anti-reflection layer formed using the Al.sub.2 O.sub.3 layer provides excellent permeability and excellent adhesiveness. However, as aging of anti-reflection layer progresses, the output power of laser light becomes lowered relatively rapidly.